Bobby-soxers, beware: The Lettermen are back, serenading audiences with sentimental favorites in their sweet, sanguine style, a combination of cable-knit harmonies and cashmere vocals that's made women swoon for more than four decades. Over the years, as members left and were replaced, the group's popularity continued, and if some of that was owing more to American Bandstand nostalgia than the Lettermen's latest material, the legacy was enough to carry over seventy albums and countless singles. Of the original threesome, only Tony Butala remains, and he won't be retiring anytime soon. A childhood member of the legendary Mitchell Boys Choir, Butala cemented his lifelong commitment to music in 1998, when he opened the Vocal Group Hall of Fame and Museum -- forty years after the Lettermen's debut. Butala and his bandmates have a few tricks up their sleeves for this tour, with some contemporary ballads -- including the Titanic theme song -- added to the set list. But there will still be plenty of classics (think "Theme From a Summer Place" and "Put Your Head on My Shoulder") plus a couple of too-syrupy numbers from these clean-cut crooners in crewnecks. Wear your heart on your sweater sleeve when the Lettermen play Boettcher Concert Hall this Tuesday (a school night!), December 21.